With the trend to continue to miniaturize semiconductor integrated circuits to achieve submicron feature sizes, photolithography has become one of the most critical steps in semiconductor manufacturing. The goal of photolithography in establishing the horizontal dimensions of the various devices and circuits is to create a pattern which meets design requirements as well as to correctly align the circuit pattern on the surface of the wafer.
As line widths shrink smaller and smaller in submicron photolithography, the process to print lines and contact holes in photoresist becomes increasingly more difficult. Photoresists have been developed to keep pace with the industry's need to print narrower lines with fewer defects. The selection of the photoresist must be made on whether the photoresist has the capability of producing the design dimensions. A thinner resist layer will generally produce narrower lines. However, the resist must simultaneously be thick enough to act as an etchant barrier and be free of pinholes.
Exposure light sources are chosen in photolithography based upon the characteristics of the photoresist. Standard production exposure tools used to print contact holes may limit how small the holes can be made. One problem with standard exposure tools is in the auto focus mechanism used to pattern a wafer. The exposure tools, when used in conjunction with thick photoresists have a small depth of focus so that light focused on the top of the photoresist will be out of focus near the bottom of the photoresist.
Production tools with light sources having longer wavelengths also create negative optical effects such as diffraction. Diffraction reduces the resolution of an image in the photoresist causing poor image definition. Image resolution, known as latent image decay, may also be reduced through the addition of chemicals to the photoresist.
The focus problem of standard production exposure tools causes poor control of the pattern imaged into the photoresist and the light needed to expose the photoresist at the bottom of the contact hole nearest the wafer. An increase in the exposure light dose to overcome this focus problem allows incident light to expose areas of the photoresist that are not targeted for exposure. Any increase in exposure time to compensate for the focus problem cannot control the incident light, resulting in poor photoresist sidewall profiles. These profiles generally result in an opening too large at the top of the photoresist causing poor circuit overlay to the underlying wafer.
An additional concern in printing submicron contacts is the resultant angle of the step from the top of the photoresist to the bottom of the contact hole. If the angle is too steep, subsequently deposited metal may be too thin over the step. One solution to this problem is to do a wet etch followed by a dry etch to achieve sloped sidewalls at the top of contact openings. Wet etches, however, have been limited to feature sizes greater than 3 microns.
It would be desirable to provide a method for printing small contact holes using a thin photoresist while protecting the integrated circuit during the etching process. It would be further desirable for such fabrication technique to provide unique etched contact profiles for use with small device geometries using dry etch techniques. It would be further desirable for such method to be compatible with current process technologies.